In mechanical punch presses, a crank mechanism is commonly used as a slide driving mechanism that converts a rotating operation of a motor into an elevating or lowering operation of a ram. Further, a flywheel is used, and a clutch is let in or released to rotate or stop the flywheel to drive or stop the ram. With the crank mechanism, curves for the elevating and lowering speeds of the ram are symmetric with respect to a bottom dead center. The lowering speed is thus the same as the elevating speed. However, for general press working including punch working, the ram preferably moves at lower speed during lowering in order to make the lowering operation silent or because of a requirement for a press load. However, the elevating operation is not particularly limited and is thus preferably faster. With a crank mechanism in which the lowering speed is the same as the elevating speed, it takes more time than required to achieve elevation. This increases a cycle time for punch working.
Recently, apparatuses have been proposed which use a servomotor as a driving source to elevate and lower the ram via a crank mechanism without using any flywheels. The servomotor can freely change the speed of the ram during its stroke and can increase its lowering speed while reducing its elevating speed. However, the capabilities of the motor depend on its rotation speed. The motor must be operated within the range of the optimum motor rotation speed according to the characteristics of the motor. If the rotation speed of the motor is controlled so that the lowering speed differs from the elevating speed, it is impossible to make full use of capabilities of the motor. A large-sized motor is required to increase the elevating speed while obtaining a required press load.
The applicant thus examined various slide mechanisms in order to select an appropriate slide mechanism that enables the ram to lower at a low speed while elevating at a high speed.
A link press has long been used as a slide mechanism used for a press device for plastic forming such as cold extrusion or upsetting of metal (for example, the Examined Japanese Patent Application Publication (Tokkou-Hei No. 3-42159). The link press comprises a pivoting link connected to a crank pin of a crank mechanism and to which a connecting rod and a restraining link are connected. The crank shaft is driven by a motor via a flywheel. With this link press, the restraining link serves to characterize the operation of the ram so that the ram lowers at a low speed and elevates fast.
However, the conventional link press is used to improve the quality of plastic forming such as cold extrusion by utilizing its very slow lowering operation performed near a bottom dead center. Thus, no conventional link presses have been applied to a punch press for which operational characteristics different from those for plastic forming are required. Further, the conventional link press is provided with a flywheel that stores output power from the motor as inertia energy. Consequently, it is difficult to properly control the conventional link press easily.
It is thus an object of the present invention to provide a motor driven link press which enables working with a heavy press load and also enables a working cycle time to be improved even when a motor with a relatively low output power is used and which can be properly controlled easily.
It is another object of the present invention to freely control an operation speed to accomplish various types of working while making use of advantages of the link press.
It is yet another object of the present invention to ensure punching scraps are dropped when the link press is applied to a punch press.